1. Field of the Invention
The present invention relates to a sensor diagnostic method and apparatus for diagnosing each sensor of a sensor system.
2. Description of the Related Art
Nowadays, data about an amount of time (a travel time) required to move from one place to another is provided on major roads as road information. Japanese Laid-open Patent Publication No. 11-110684 discusses a sensor system for acquiring such data regarding a travel time, for example.
In addition, Japanese Laid-open Patent Publication No. 2006-244338 discusses a sensor system for locating a specific product, for example.
As shown in FIG. 1, the sensor system for acquiring data about a travel time includes sensors (vehicle license plate readers) 1a, 1b, and 1c allocated in a plurality of locations on the road, and a center apparatus 3 connected to the sensors 1a, 1b, and 1c through a network 2 and gathering number data (read vehicle license number and read time) output from the sensors 1a, 1b, and 1c. 
The center apparatus 3 determines a travel time of each section on the basis of number data sent from the sensors 1a, 1b, and 1c, by obtaining a difference between the shot times of the same vehicle license number at respective locations where the sensors 1a, 1b, and 1c are allocated.
As shown in FIG. 2, the system for locating a specific product includes an RFID (radio frequency identification) tag 6 attached to a product 5 to be managed, sensors (RFID readers) 7a, 7b, and 7c allocated in a plurality of locations within a roaming area of the product, and a center apparatus 9 connected to the sensors 7a, 7b, and 7c through a network 8 and gathering ID data (read ID and read time) output from the sensors 7a, 7b, and 7c through the network 8. The center apparatus 9 grasps a current location and roaming history of each product by obtaining read times of a product with the same ID at respective locations where the sensors 7a, 7b, and 7c are allocated.
Conventional systems operate on the presumption that sensors operate normally as expected and identification data of a specific product within a predefined distance from a sensor can be surely acquired without any loss. However, in fact, a sensor may output wrong data or lose data due to aging, a change in installation environments, and the like.
For example, a vehicle license plate reader, which reads a vehicle license plate from a video image captured with a camera, may not correctly read a vehicle license plate if a camera lens gets fogged or soiled during operation. In this case, an output result may involve an error or loss.
In addition, an RFID reader may not correctly read an ID when some object shielding or reflecting a radio wave is allocated within a sensing area or a direction of an antenna is changed during operation. In this case, an output result may involve any loss.
As discussed above, an abnormal operation of a sensor during operation of a system causes an abnormal operation of the system. Thus, it is necessary to check whether each sensor operates normally in order to normally operate the system.